1. Field of the Invention
The present invention relates to a sheet transport direction switching device for switching a transport direction of a sheet to be transported, and an image forming apparatus incorporated with the sheet transport direction switching device.
2. Description of the Related Art
Conventionally, there is known a sheet transport direction switching device (e.g. a sheet transporting and treating device in JP Hei 9-86759A) for use in a sheet transport system in an image forming apparatus, as recited in JP Hei 9-86759A. The sheet transport direction switching device is constructed in such a manner that after an image forming operation is performed by an image forming apparatus, a discharge destination of a sheet carrying a toner image on a surface thereof is switched over between a discharge tray, and a switchback path for a double-side printing operation, wherein a rotary guide member (corresponding to a switching gate in JP Hei 9-86759A) is provided at a branching position of the two destinations.
The rotary guide member is constructed in such a manner that four guide plates are mounted between a pair of circular side plates disposed opposite to each other with a distance slightly larger than a sheet width, and that rotating shafts extend in directions opposite to each other from center positions of the circular side plates, respectively.
Three guide passages i.e. a middle straightforward guide passage, and two inverse guide passages defined at both sides of the middle straightforward guide passage are defined between each opposing pair of the guide plates. A sheet transported toward the rotary guide member is selectively passed through one of the guide passages depending on a rotated amount of the rotary guide member with respect to a reference phase thereof. Thereby, the sheet is discharged to a predetermined destination.
The rotary guide member is integrally rotated about an axis of rotation thereof by a stepping motor which is drivingly rotated depending on the number of pulses of a pulse signal. With this arrangement, the position of the rotary guide member is defined, in other words, a discharge destination of a sheet transported to the rotary guide member is determined.
A stepping motor is constructed in such a manner that a so-called detent torque i.e. a retention torque for fixedly holding the stepping motor at a rotated position thereof is generated by bringing the stepping motor to a non-energizing state by setting a current flowing through a coil of the stepping motor to zero. Bringing the stepping motor to a non-energizing state after the rotary guide member is fixed at a predetermined rotated position by the stepping motor enables to fixedly position the rotary guide member by a detent torque.
The rotary guide member is operable to bend a sheet transport direction by abutment of a lead end of a sheet transported to the rotary guide member against a guide plate constituting a guide passage at an entrance of the rotary guide member. In this arrangement, a rotation torque larger than a detent torque may be generated when a sheet is abutted against the guide plate, with the result that the rotary guide member may be rotated, and proper sheet transport may be obstructed.